Facilities such as an activated-sludge treatment facility and a coagulating-sedimentation treatment facility have employed, in general, sedimentation in which a solid-liquid separation tank (a settling tank) is used as means for separating sludge mixture into treated water and sludge. In such sedimentation, a sludge-blanket filtration method is employed in which a sludge zone (a sludge blanket layer) is formed in a sedimentation tank to obtain treated water having good quality by efficiently removing suspensoids and fine SS from the sludge mixture. The sludge mixture is made to flow into a portion below the sludge zone and to flow through the sludge zone, whereby the suspensoids and fine SS in the sludge mixture are filtered and separated from the mixture.
According to patent literature 1 and patent literature 2, floc-containing raw water is made to flow into a settling tank via an inlet provided in one side face of a tank body of the settling tank, and concentrated sludge is made to flow out of the tank body via an outlet provided in another side face of the tank body. According to patent literature 2, the mixture in the tank body is stirred by a stirring wheel.
According to patent literature 3, a reticular member is provided horizontally near a middle part of a tank body of a settling tank, floc-containing raw water is made to flow into a portion below the reticular member, and a sludge blanket is formed on the upper side of the reticular member. According to patent literature 3, some of the sludge in an upper-layer portion of the sludge blanket is made to flow out of the tank body via an outlet provided in a side face of the tank body.
According to patent literature 4, floc-containing raw water is made to flow from a distributor into a tank body, sludge accumulated in a lower portion of a sludge blanket is collected by using a rake provided below the distributor, and concentrated sludge is extracted from the bottom center of the tank body.
Compared with the settling tank according to patent literature 4 employing a concentrated-sludge-bottom-extracting method, the settling tank according to any of patent literatures 1 to 3 employing a side-discharge method in which sludge in the upper-layer portion of the sludge blanket is made to flow out of the tank body through a side face of the tank body has an advantage in that the deterioration in the quality of treated water due to a sudden rise of the blanket surface level is less likely to occur.